Elastically tailored composite laminates are laminates that have been designed with specifically tailored stiffness parameters. These parameters can be sized to give an out-of-plane response to an in-plane loading, resulting in deformation modes not found in conventional isotropic materials. Two examples of elastically tailored composite laminates are those that exhibit bend-twist coupling and those that exhibit extension-twist coupling. A bend-twist coupled laminate will twist about its longitudinal axis when a lateral bending moment is applied thereto. Such laminates have application in fixed wing structures of aircraft where an increase in wing loading may produce a wing-tip wash-in to help prevent tip stall.
An extension-twist coupled laminate will twist about its longitudinal axis when an axial force is applied thereto. Extension-twist coupled laminates have application in rotary wings such as the rotors of helicopters where an increase in centrifugal load can result in an increase or decrease in the pitch angle of the rotor blade. Pretwisted strips made of isotropic materials such as metals exhibit extension-twist coupling as they tend to untwist when subjected to axial force. The present invention is concerned with these extension-twist coupled strips and laminates.
When designing extension-twist coupled specimens, it is imperative that the stiffness characteristics of the specimens be determined accurately. That is, it must be known accurately how much twist is induced in the specimen when a known axial load is applied. In the past, experimental methods to determine the extension-twist characteristics of a specimen have relied on specialized test apparatuses built for a narrow field of use. One such apparatus used a suspended weight applied through a thrust bearing and reflected laser light beams to determine the twist distribution of the test specimen. While the measurements were accurate, higher loads were not attempted and the apparatus required a good amount of support equipment.
In instances where a higher axial load has been required to induce the twist to be measured, suspended weight measuring machines are not feasible. In these instances, conventional servohydraulic bi-axial testing machines may be used. Such machines function through the use of hydraulic actuators and load cells. For a tension-torsion servohydraulic testing machine, there are two hydraulic actuators, one for axial load application and one for torque application. Similarly, there are two load cells; one to measure the axial load and one to measure the torque. Closed loop control is made by comparing the load cell feedback to the commanded control signal. With a tuned system, axial load and torque can be applied and controlled. In the case of the extension-twist coupled specimens, it is desirable to know what the twist response is due to an applied axial load. Implicit in that statement is that at least one end of the test specimen is free to twist. A bi-axial test machine would be programmed to load axially while maintaining the zero torque condition required by free twist. However, in the case of specimens with low torsional rigidity, such as extension-twist coupled laminates, the torsional rigidity is lower than the noise threshold for torsion load cells and the torque actuator does not receive the correct feedback signal, resulting in a perceived stiffening of the specimen. An accurate survey of the stiffness properties is therefore not possible.
Thus, there exists a need for an improved method and apparatus for measuring the twist versus axial load characteristics of an extension-twist coupled specimen. Such a method and apparatus should allow free twisting as an axial load is applied with zero or negligible torsional resistance to the twisting motion because of friction. Further, the resistance should remain zero or negligible as ever increasing axial force is applied, even to the point where the specimen experiences actual structural failure. Preferably, the apparatus should incorporate reliable, accurate, and repeatable means for measuring the amount of twist induced in the specimen as a function of the applied axial force. The device should be simple in construction and operation, should be able to be produced and marketed for an affordable cost, and should be directly usable in its own load frame or incorporated with an existing load frame. It is to the provision of such a method and apparatus that the present invention is primarily directed.